The present invention relates to data communications, and more particularly, to coherent demodulation of coherently transmitted phase modulated data bursts on rf channels subject to Rician fading.
Satellite-based mobile communication systems are being developed for economically providing mobile communication service on a global basis. As shown in FIG. 1, a satellite based communication system 1 may have a relatively large geographical coverage area because the satellite's antennas may be configured to cover areas as large as a continent.
Unlike existing cellular mobile communication systems, satellite-based mobile communication systems are subject to Rician channel fading. Rician channel fading results from a combination of fading due to atmospheric effects and Rayleigh fading which is inherent in mobile communication systems. For various reasons, including those relating to Rician fading, digital methods are generally implemented in satellite-based mobile communication systems.
In a typical implementation, an audio voice signal is converted to a digital signal and transmitted through the atmosphere 3 such that a portion of the signal is captured by a satellite receiver. The satellite 4 relays the digital signal to a ground-based station 4 for connection with land-based telephone lines or to another mobile station 2. Because an audio voice signal has a relatively small bandwidth, several signals are often time and/or frequency multiplexed within a larger bandwidth of the satellite's communications channel. Time multiplexing, for example, involves sending the digital audio signals as relatively short data bursts. Further, phase modulation is often used to encode the digital data signal.
The mobile station, and in some systems, the satellite, must decode the digital signal after it is transmitted across the data channel. The characteristics of the data channel, including phase fluctuations due to noise or Rician fading, alter the phase encoding of the transmitted data. Existing decoding methods give rise to phase ambiguities due to cycle slips and, accordingly, phase-modulated data is generally differentially encoded to resolve the phase ambiguities in the carrier phase estimation process. Differential encoding results in a detection penalty of at least 1 dB, which is significant under the limited power constraints of mobile communication systems.
Accordingly, there exists a need for mobile and satellite stations that efficiently demodulate coherently transmitted phase-modulated data bursts on rf channels subject to Rician fading. The present invention satisfies this needs.